Field of the Invention
The present invention relates to a heat treatment apparatus and a heat treatment method for heating a thin plate-like precision electronic substrate such as a semiconductor wafer and a glass substrate for a liquid crystal display device (hereinafter referred to simply as a “substrate”) by irradiating the substrate with a flash of light.
Description of the Background Art
In the process of manufacturing a semiconductor device, impurity doping is an essential step for forming a pn junction in a semiconductor wafer. At present, it is common practice to perform impurity doping by an ion implantation process and a subsequent annealing process. The ion implantation process is a technique for causing ions of impurity elements such as boron (B), arsenic (As) and phosphorus (P) to collide against the semiconductor wafer of silicon with high acceleration voltage, thereby physically implanting the impurities into the semiconductor wafer. The implanted impurities are activated by the subsequent annealing process. When annealing time in this annealing process is approximately several seconds or longer, the implanted impurities are deeply diffused by heat. This results in a junction depth much greater than a required depth, which might constitute a hindrance to good device formation.
In recent years, attention has been given to flash lamp annealing (FLA) that is an annealing technique for heating a semiconductor wafer in an extremely short time. The flash lamp annealing is a heat treatment technique in which xenon flash lamps (the term “flash lamp” as used hereinafter refers to a “xenon flash lamp”) are used to irradiate a surface of a semiconductor wafer with a flash of light, thereby raising the temperature of only the surface of the semiconductor wafer implanted with impurities in an extremely short time (several milliseconds or less).
The xenon flash lamps have a spectral distribution of radiation ranging from ultraviolet to near-infrared regions. The wavelength of light emitted from the xenon flash lamps is shorter than that of light emitted from conventional halogen lamps, and approximately coincides with a fundamental absorption band of a silicon semiconductor wafer. Thus, when a semiconductor wafer is irradiated with a flash of light emitted from the xenon flash lamps, the temperature of the semiconductor wafer can be raised rapidly, with only a small amount of light transmitted through the semiconductor wafer. Also, it has turned out that flash irradiation, that is, the irradiation of a semiconductor wafer with a flash of light in an extremely short time of several milliseconds or less allows a selective temperature rise only near the surface of the semiconductor wafer. Therefore, the temperature rise in an extremely short time with the xenon flash lamps allows only the activation of impurities to be achieved without deep diffusion of the impurities.
Heat treatment apparatuses which employ such xenon flash lamps are disclosed in U.S. Patent Application Publication Nos. 2009/0067823 and 2009/0103906 in which an insulated gate bipolar transistor (IGBT) is connected to a light emitting circuit for a flash lamp to control the light emission from the flash lamp. In the apparatuses disclosed in U.S. Patent Application Publication Nos. 2009/0067823 and 2009/0103906, a predetermined pulse signal is outputted to the gate of the IGBT to define the waveform of current flowing through the flash lamp, thereby controlling the light emission from the lamp. This achieves the adjustment of the temperature profile of the front surface of a semiconductor wafer.
In the apparatuses disclosed in U.S. Patent Application Publication Nos. 2009/0067823 and 2009/0103906, 30 IGBTs are provided in a one-to-one correspondence with 30 flash lamps, and a common pulse signal is outputted to the 30 IGBTs. Thus, currents having the same waveform flow through the 30 flash lamps, so that the 30 flash lamps emit light in a similar fashion.
Even if a plurality of flash lamps emit light in a similar fashion, an actual flash lamp annealer has a problem such that nonuniformity in illuminance coming from an apparatus configuration problem results in the nonuniform in-plane temperature distribution of a semiconductor wafer during flash irradiation. In general, a semiconductor wafer W shows a tendency to be lower in temperature in a peripheral portion thereof than near a central portion thereof.